(1) Field of the Invention
The present invention relates to an apparatus for optoelectrically detecting floating particles in ambience by using a laser beam, and in particular, relates to such an optoelectrical particle detection apparatus which can be advantageously used to detect floating particles in a closed ambience such as a clean room for the production of semi-conductors, and a vacuum chamber for the formation of a thin film by, for example, a vacuum evaporation process, a sputtering process and a chemical vapor deposition process.
(2) Description of the Related Arts
An optoelectrical particle detection apparatus is well known, as disclosed in, for example, U.S. Pat. No. 4,655,592 and U.S. Pat. No. 4,422,761. U.S. Pat. No. 4,655,592 is directed to an apparatus for optoelectrically detecting particles on a surface of a substrate, wherein a light emitted from a light source is focused on the substrate surface as a small spot through an optical system, and wherein a light detector such as a photomultiplier tube for receiving a light scattered from the small light spot due to a presence of a particle therewithin is provided. U.S. Pat. No. 4,422,761 is directed to an apparatus for optoelectrically detecting floating particles in air, wherein a light emitted from a light source is focused on an inspection point through an optical system, an air flow including particles to be detected being continuously passed through the inspection point, and wherein a light detector such as a photomultiplier tube for receiving a light deflected or scattered from the inspection point due to the presence of particles included in the air flow is provided.
As apparent from the foregoing, in an optoelectrical particle detection apparatus of the types disclosed in U.S. Pat. Nos. 4,655,592 and 4,422,761, the zone in which the presence of particles can be detected is very restricted, and thus it is impossible or very difficult to effectively detect the presence of particles over a large area. In addition, the apparatus of U.S. Pat. No. 4,655,592 can not be adapted for the detection of particles in a vacuum chamber for the formation of a thin film element, as mentioned above, because it is impossible to use a medium such as an air flow entraining the particles passing the inspection point.
Unexamined Japanese Patent Publication No. 61-240645 discloses an optoelectrical particle detection apparatus wherein a scanning laser beam is used to widen a zone in which the presence of particles can be detected, and wherein a TV camera is provided for receiving light scattered from the scanning beam due to the presence of particles in the scanning zone. In a detection apparatus of this type, however, since particles to be detected have a higher velocity, the particle detection probability becomes lower, and may be equivalent to that of detecting particles by using a non-scanning or single static laser beam. Namely, when particles to be detected have a very high velocity, it is meaningless to widen the detection zone by using a scanning beam.
Also well known is an optoelectrical particle detection apparatus wherein a strong laser beam is used to detect particles, with a high sensitivity. In this apparatus, which is commercially available, a strong laser beam between resonance mirrors of a laser generator is used for particle detection. In a detection of this type, however, the zone for detection of particles is also very restricted due to use of the single laser beam, and the application of this type of particle detection is limited because the detection zone must be provided in the laser generator.
Unexamined Japanese Patent Publication No. 61-243345 discloses another particle detection apparatus wherein a strong laser beam is used for the detection. In this apparatus, a laser generator is provided with an outside second resonator in which an output mirror of the laser generator is utilized as one of the resonance mirrors, and thus a strong laser beam obtained in the second resonator is used for the particle detection. Nevertheless, this apparatus suffers from the same defects as the strong laser beam detection apparatus mentioned above.
In another well known optoelectrical particle detection apparatus including a pair of parallel plane reflectors, a semiconductor laser beam is multi-reflected to form a laser beam curtain of the multi-reflected beam segments as a zone in which floating particles can be detected, whereby the floating particles can be detected over a wide area with a high probability by scattered sensing light due to the presence of particles in the laser beam curtain. A detection apparatus of this type is commercially available from High Yield Technology Inc. of the United States of America.
In this detection apparatus, since it is very difficult to multi-reflect the laser beam at a very close pitch, it is substantially impossible to obtain a laser beam curtain having a uniform light intensity. Accordingly, although the presence of particles is detected in the laser beam curtain, the size of the detected particles cannot be determined because the intensity of the scattered light from the same size of particle differs in accordance with the location of the particle in the laser beam curtain, due to the nonuniform light intensity thereof.
In the multi-reflection type apparatus as mentioned above, the parallel plane reflectors must be precisely positioned with respect to each other to obtain an accurate parallel relationship therebetween before the laser beam curtain can be formed between the plane reflectors, but this positioning of the parallel plane reflectors is very complex and difficult because at least one of the plane reflectors must be angularly adjusted around two axes perpendicular to each other.